1. Field of the Invention
This disclosure describes a novel motion picture projection screen with high gain and conservation of polarization for good 3D or stereoscopic viewing over a broad viewing angle. This front-projection screen works well with existing screen manufacturing techniques and methods for hanging screens in motion picture theaters.
2. Background of the Invention
The motion picture industry, like many industries, is tradition-bound and technical innovation tends to come from the outside. There are numerous examples of this, such as the introduction of sound, color, widescreen, and 3D. Any change to the existing motion picture infrastructure has to be carefully measured in terms of its economic benefits compared with established industry methods. For any innovation to prevail, the innovation should cooperate with the existing infrastructure by making relatively incremental changes to industry methods.
Certain established methods of manufacture of motion picture screens exist, and as important, specifications and methods exist that have to be followed in the motion picture theater in order to obtain the exhibitors' acceptance. Although there are good reasons for exhibitor acceptance of improved motion picture screens, ongoing concerns remain, such as the sound system's requirements, the ability to clean the screen, the cost of the screen, the means for hanging or installing the screen, and a host of other considerations, not the least of which is the image quality of the screen good for both two dimensional and three dimensional (2D and 3D) projection. The screen should have high contrast, an unobtrusive surface, and even illumination should be provided across the screen surface from any seat in the house.
Of particular concern in projecting stereoscopic motion pictures is the conservation of polarization. Screens that conserve polarized light must virtually always have a metallic surface, typically of painted on aluminum. The properties of that aluminum surface are to a large extent determined not only by the particulate size of the aluminum pigment, but also the binder used, and the method of application. Of particular concern is that the screen preserves the properties of polarized light for stereoscopic image selection. If depolarization occurs, the result will be crosstalk, wherein a portion of the unwanted perspective view is observed by each eye of a user. Crosstalk is undesirable, and detracts from the enjoyment of the stereoscopic movie by reducing the depth effect and causing viewer fatigue.
Motion picture screen performance for stereoscopic projection has been analyzed for decades. A lenticular or lenticulated screen generally must provide a good result in terms of having even illumination over a wide viewing angle while conserving polarization. The term “lenticulated,” or “lenticular,” implies that a refractive lens element is used. Specific to front-projection screen design, the term refers to rib-like structures that reflect, rather than refract, light.
Several previous designs for lenticules exist. Motion picture screens are invariably rectangular in shape. Certain designs have employed lenticules of stepped ridges with straight surfaces, or beads with a diffusing surface. Generally, previous developers have recognized the balance between specular, or highly reflective, and diffuse surfaces. A completely specular surface is a mirror, and a mirror has a very bright and small “hot spot,” where a hot spot is the reflection from the reflective surface that provides a glare to a user Texturing of the aluminum screen surface can be added to make the screen semi-specular.
Other lenticular screen designs use different ribbed configurations where ribs, or lenticules, are employed, almost invariably running in the vertical direction, so that the projected light lost from extreme side angles of the screen can be deployed in the direction of the audience. In these designs the ribbed surface resembles that of a washboard or corduroy fabric. The location where the ribs intersect is a boundary line that can be called the “boundary axis” of the lenticular, or ribbed, or ridged, surface. Typically the boundary axis is oriented vertically, or parallel to the vertical edge of the screen in the case of a rectangular screen.
Certain screen designs employ a lenticular structure for increasing what is termed “screen gain.” In many circumstances, some screen gain is desirable. Estimates are that only one third of the volume in space in front of a projection screen contains seating, meaning two-thirds of the projected light is wasted or unavailable for the aggregate eyes of the audience. Note that a motion picture screen cannot amplify light, but can only take the light projected onto the screen and reflect the light.
The more diffuse the surface of the screen, the more uniform is the intensity of reflected illumination as a function of angle. Such a screen is described as a screen with a matte surface, and a perfect matte screen for the purposes of this discussion can be described as one having a gain of 1.0. Such a screen has many advantages, not the least of which is that the screen provides even illumination to any seat in the house. In other words, even if an audience member is sitting far to the side, the brightness of the screen remains constant over the entire surface of the screen. A screen with a gain of 1.0 means that every audience member sees an image having the same brightness. However, a screen with a gain of 1.0 wastes a significant amount of light reflected to the ceiling, floor, and sides of the auditorium. A screen with a gain of 1 may be desirable, especially for a wide auditorium or one with a balcony or balconies, but for most current theater designs a significant amount of light energy is wasted.
Screen designers have attempted to overcome light losses by using so-called lenticular screens. The job of the lenticules or ribs is to present more surface area in a horizontal direction with a surface promoting reflecting properties in the horizontal. Light is gathered in the horizontal and reflected toward the seats in the audience where the light is needed. The side light which would be wasted is gathered and reflected to a more appropriate area of the theater, where people are seated. Such a screen is described as having “gain.” Screen performance or gain can be measured with a photometer and compared with a matte screen, and a screen using such a lenticular design can exhibit decent gain. In general, screens having a gain much higher than 2.0 may result in hot-spotting. As noted, the ultimate hot spot results in a specular screen that is a mirror, exhibiting a very small hot spot.
Screens have been designed that are semi-specular in an attempt to reduce the hot spot. The ribs or lenticules act as a kind of virtual curvature because the ribs or lenticules are often used on flat screen surfaces. Hot-spotting can also be mitigated through screen cylindrical curvature, usually as a concave surface facing the audience.
One screen exhibiting strong gain, conservation of polarization, and spreading the light over well defined angle characteristics is the design used to produce the Kodak Ektalite screen of Chandler. The screen had an unintended benefit of having excellent polarization conservation characteristics. The Kodak Ektalite screen, now out of production, was a concave screen, the inside section of a sphere. The screen had the rectangular shape as required for motion picture and slide projection and was a rigid solid screen, having an aluminum foil coating applied to its concave surface. The aluminum foil coating had a bark-like texture, which served to soften the specular nature of the reflections. The screen had extremely high gain and a radius of curvature approximately 4.5 times the width of the front surface of the screen.
Two versions of such a screen have been described. One is built on a rigid surface of lathing, for example, covered with plaster, sanded smooth, and then sprayed with aluminum paint. The other is similar to a design known in the industry as a Torus screen, in which a membrane-like surface is coated with aluminum and hung on a frame concave in the vertical and horizontal directions. The screen is mounted on a boxlike structure with an exhaust fan that creates low pressure so that the screen will assume a surface that is the inside section of a sphere. Such a screen has excellent properties with regard to gain, and also for the conservation of polarized light if a proper aluminum surface is used. Although an ellipsoid has been recommended as the preferred section for the screen surface, the inside section of a sphere can approximate an ellipsoid. A screen whose vertical and horizontal sections are sections of a circle can provide similar benefits.
Problems exist with respect to the Kodak Ektalite type screen since a solid screen has to be built in place in the theater. Using a Kodak Ektalite type screen in a theater would significantly depart from exhibitors' current practices. Current screens are relatively easy to ship and assemble. They are rolled into a cylinder, like a rug, for shipment and assembled on a frame with cords attached to the frame pulling on the screen's grommets. The Chandler Kodak Ektalite design generally must be built in place, or assembled from sections, or some such technique that significantly departs from currently accepted theater screen implementation practice.
In addition, significant issues exist with a solid screen due to speaker placement issues. One of the important practices in the art of motion picture projection is to use a perforated screen, i.e. a screen with a regular pattern of small holes. Loudspeakers are placed behind the screen containing the perforations allowing sound to pass through the screen. This arrangement gives the audience the sensation that the sound is located in the same direction as the projected image. Theater owners have concerns over a non-perforated screen, as placement of speakers in other locations is perceived to compromise the audience perception of directionality of sound. Another problem with the Kodak Ektalite design concerns sound, but of a different nature. The concave shape of the screen and its smooth surface gives rise to audience members hearing the sound of conversations reflected from distant parts of the auditorium.
This approach to screen design, while having benefits, violates concerns described above: (1) such a screen on a theater scale involves an extraordinary screen manufacturing procedure, (2) with regard to the method of installation or hanging, such a screen is unlike other screens that are used in motion picture theaters, and (3) such a screen requires substantial changes in the deployment of theater loud speakers. Because exhibitors wish to have a proven product, such a radical design has led to limited acceptance of new screen designs.
In an attempt to overcome the difficulties with regard to a screen with a compound curvature surface, inventors have turned their attention to replicating this surface by means of special lenticules or ribs embossed or applied to the screen surface in order to provide the benefits that have been described above but without the necessity of actually having a screen with a compound surface. These designs include a Fresnel-like arrangement of ribbing that will appropriately direct light, as well as a screen material that can produce various surfaces and various types of lenticules. Another approach uses a similar Fresnel-like lens shape for reflection and which, in effect, can take the place of a compound screen. These are typically flat screens containing special ribbings or lenticules.
Unfortunately, these screens can be difficult to manufacture, and in terms of installation in a particular theater, each screen requires a unique optical design. Such a consideration would probably rule them out in terms of commercial deployment.
It would be desirable to offer a screen that overcomes certain screen design issues present in the prior designs.